Air Conditioning Chapter 7.pdf

CHAPTER 7

AIR CONDITIONING

Learning Objectives: Understand the principles of air conditioning and the operation

of basic air-conditioning systems. Recognize the characteristics and procedures

required to install, operate, and maintain air-conditioning systems

Air conditioning is the simultaneous control of

temperature, humidity, air movement, and the quality

of air in a conditioned space or building. The intended

use of the conditioned space is the determining factor

for maintaining the temperature, humidity, air

movement, and quality of air. Air conditioning is able

to provide widely varying atmospheric conditions

ranging from conditions necessary for drying

telephone cables to that necessary for cotton spinning.

Air conditioning can maintain any atmospheric

condition regardless of variations in outdoor weather.

TEMPERATURE

Temperature, humidity, and air motion are

interrelated in their effects on health and comfort. The

term given to the net effects of these factors is effective

temperature. This effective temperature cannot be

measured with a single instrument; therefore, a

psychrometric chart aids in calculating the effective

temperature when given sufficient known conditions

relating to air temperatures and velocity.

Research has shown that most persons are

comfortable in air where the effective temperature lies

within a narrow range. The range of effective

temperatures within which most people feel

comfortable is called the COMFORT ZONE. Since

winter and summer weather conditions are markedly

different, the summer zone varies from the winter

zone. The specific effective temperature within the

zone at which most people feel comfortable is called

the COMFORT LINE (fig. 7-1).

This chapter explains the following subjects as they

pertain to air conditioning: principles of air

conditioning, heat pumps, chilled-water systems,

periodic maintenance, cooling towers, troubleshooting,

automotive air conditioning, and ductwork.

PRINCIPLES OF AIR CONDITIONING

Learning Objective: Understand the basic principles

of temperature, humidity, and air motion in relation to

air conditioning.

HUMIDITY

Air at a high temperature and saturated with

moisture makes us feel uncomfortable. However, with

the same temperature and the air fairly dry, we may

feel quite comfortable. Dry air, as it passes over the

surface of the skin, evaporates the moisture sooner

than damp air and, consequently, produces greater

cooling effect. However, air may be so dry that it

causes us discomfort. Air that is too dry causes the

surface of the skin to become dry and irritates the

membranes of the respiratory tract.

HUMIDITY is the amount of water vapor in a

given volume of air. RELATIVE HUMIDITY is the

amount of water vapor in a given amount of air in

comparison with the amount of water vapor the air

would hold at a temperature if it were saturated.

Relative humidity may be remembered as a fraction or

percentage of water vapor in the air; that is, DOES

HOLD divided by CAN HOLD.

Air conditioning is the process of conditioning the

air in a space to maintain a predetermined temperature-

humidity relationship to meet comfort or technical

requirements. This warming and cooling of the air is

usually referred to as winter and summer air

conditioning.

Here, you are introduced to the operating principles

of air-conditioning systems, the environmental factors

controlled by air conditioning, and their effects on

health and comfort. Refrigerative air conditioners and

general procedures pertaining to the installation,

operation, and maintenance of these systems are

examined. Also, the operation and maintenance of the

controls used with these systems are explained.

7-1

Figure 7-1.—Comfort zones and lines.

Relative humidity is determined by using a sling

psychrometer. It consists of a wet-bulb thermometer

and a dry-bulb thermometer, as shown in figure 7-2.

The wet-bulb thermometer is an ordinary thermometer

similar to the dry-bulb thermometer, except that the

bulb is enclosed in a wick that is wet with distilled

water. The wet bulb is cooled as the moisture

evaporates from it while it is being spun through the

air. This action causes the wet-bulb thermometer to

thermometer. Tables and charts have been designed

that use these two temperatures to arrive at a relative

humidity for certain conditions.

within which the majority of adults feel comfortable.

In looking over the chart, note that the comfort zone

represents a considerable area. The charts show the

wet- and dry-bulb temperature combinations that are

comfortable to the majority of adults. The summer

comfort zone extends from 66°F effective temperature

to 75°F effective temperature for 98 percent of all

personnel. The winter comfort zone extends from 63°F

effective temperature to 71°F effective temperature for

97 percent of all personnel.

Dew-Point Temperature

A comfort zone chart is shown in figure 7-3. The

comfort zone is the range of effective temperatures

The dew point depends on the amount of water

vapor in the air. If the air at a certain temperature is not

Figure 7-2.—A standard sling psychrometer.

7-2

Figure 7-3.—Comfort zone chart.

saturated (maximum water vapor at that temperature)

and the temperature of that air falls, a point is finally

reached at which the air is saturated for the new and

lower temperature, and condensation of the moisture

begins. This is the dew-point temperature of the air for

the quantity of water vapor present.

As the amount of moisture in the air increases,

the amount of evaporation (and, therefore,

cooling) decreases. The difference between the

temperatures becomes less.

When the air becomes saturated, all three

temperatures are the same and the relative

humidity is 100 percent.

To HUMIDIFY air is to increase its water vapor

content. To DEHUMIDIFY air is to decrease its water

vapor content. The device used to add moisture to the

air is a humidifier, and the device used to remove the

moisture from the air is a dehumidifier. The control

device, sensitive to various degrees of humidity, is

called a HUMIDISTAT.

Methods for humidifying air in air-conditioning

units usually consist of an arrangement that causes air

to pick up moisture. One arrangement consists of a

Relationship of Wet-Bulb, Dry-Bulb, and

Dew-Point Temperatures

A definite relationship exists between the wet-bulb,

dry-bulb, and dew-point temperatures. These

relationships are as follows:

When the air is not saturated but contains some

moisture, the dew-point temperature is lower

than the dry-bulb temperature, and the wet-bulb

temperature is in between.

7-3

heated water surface over which conditioned air passes

and picks up a certain amount of water vapor by

evaporation, depending upon the degree of

humidifying required. A second arrangement to

humidify air is to spray or wash the air as it passes

through the air-conditioning unit.

Often water sprays are used to recondition the air

by washing and cleaning it. These sprays may also

serve to humidify or dehumidify the air to some extent.

In some large air-conditioning systems, air is

cleaned by electricity. In this type of system, electrical

precipitators remove the dust particles from the air.

The air is first passed between plates where the dust

particles are charged with electricity; then the air is

passed through a second set of oppositely charged

plates that attract and remove the dust particles (fig.

7-4). This method is by far the best method of air

cleaning, but the most expensive.

During the heat of the day, the air usually absorbs

moisture. As the air cools at night, it may reach the dew

point and give up moisture, which is deposited on

objects. This principle is used in dehumidifying air by

mechanical means.

Dehumidifying equipment for air conditioning

usually consists of cooling coils within the air

conditioner. As warm, humid air passes over the

cooling coils, its temperature drops below the dew

point and some of its moisture condenses into water on

the surface of the coils. The condensing moisture gives

up latent heat that creates a part of the cooling load that

must be overcome by the air-conditioning unit. For this

reason, the relative humidity of the air entering the air

conditioner has a definite bearing on the total cooling

load. The amount of water vapor that can be removed

from the air depends upon the air over the coils and the

temperature of the coils.

CIRCULATION OF AIR

The velocity of the air is the primary factor that

determines what temperature and humidity are

required to produce comfort. (The chart in figure 7-3 is

based on an air movement of 15 to 25 feet per minute.)

We know from experience that a high velocity of air

produces a cooling effect on human beings. However,

air velocity does not produce a cooling effect on a

surface that does not have exposed moisture. A fan

does not cool the air, but merely increases its velocity.

The increased velocity of air passing over the skin

surfaces evaporates moisture at a greater rate; thereby,

cooling the individual. For this reason, circulation of

air has a decided influence on comfort conditions. Air

can be circulated by gravity or mechanical means.

PURITY OF AIR

The air should be free from all foreign materials,

such as ordinary dust, rust, animal and vegetable

matter, pollen, carbon (soot) from poor combustion,

fumes, smoke, and gases. These types of pollution are

harmful to the human body alone; however, they

include an additional danger because they also carry

bacteria and harmful germs. So, the outside air brought

into a space or the recirculating air within a space

should be filtered during air conditioning.

When air is circulated by gravity, the cold, and

therefore heavier, air tends to settle to the floor, forcing

the warm and lighter air to the ceiling. When the air at

the ceiling is cooled by some sort of refrigeration, it

will settle to the floor and cause the warm air to rise.

The circulation of the air by this method will

eventually stop when the temperature of the air at the

ceiling is the same as the temperature on the floor.

Air in an air conditioner may be purified or cleaned

by filters, air washing, or electricity.

Air may be circulated by mechanical means by

axial or radial fans. When either the axial or radial fan

is mounted in an enclosure, it is often called a blower.

Filters may be designed as permanent or

throwaway types. They are usually made of fibrous

material, which collects the particles of dust and other

foreign matter from the air as it passes through the

filter. In some cases, the fibers are dry, while in others

they have a viscous (sticky) coating. Filters usually

have a large dust-holding capacity. When filters

become dust-laden, they are either discarded or

cleaned. Permanent filters are usually cleaned.

Throwaway filters are only one-time filters and are

discarded when they become dust-laden.

Q1. What is the term given to the net effects of

temperature, humidity, and air motion?

Q2. The comfort line is the specific effective

temperature at which most people feel

comfortable. True /False

Q3. What is the term for the amount of water vapor in

a given volume of air?

Q4. What instrument is used to measure relative

humidity?

7-4

Figure 7-4.—Diagram of an electrostatic filter.

Q5. The point where water vapor condenses is called

the dew point. True /False

For example, an installation that requires re-heating of

the conditioned air must be arranged with the

re-heating coil on the downstream side of the

dehumidifying coil; otherwise, re-heating of the

cooled and dehumidified air is impossible.

Q6. What condition exists when the dry-bulb,

wet-bulb, and dew-point temperatures are the

same?

There has been a tendency by many designers to

classify an air-conditioning system by referring to one

of its components. For example, the air-conditioning

system in a building may include a dual duct

arrangement to distribute the conditioned air;

therefore, it is then referred to as a dual duct system.

This classification makes no reference to the type of

refrigeration, the piping arrangement, or the type of

controls.

Q7. What are the two types of filter designs?

Q8. What is the primary factor that determines the

temperature and humidity required for room

comfort?

AIR-CONDITIONING SYSTEMS

Learning Objective: Recognize basic types of

air-conditioning systems, and understand the operation,

maintenance, and repair methods and procedures.

For the purpose of classification, the following

definitions are used:

A complete air-conditioning system includes a

means of refrigeration, one or more heat transfer units,

air filters, a means of air distribution, an arrangement

for piping the refrigerant and heating medium, and

controls to regulate the proper capacity of these

components. In addition, the application and design

requirements that an air-conditioning system must

meet make it necessary to arrange some of these

components to condition the air in a certain sequence.

An air-conditioning unit is understood to consist

of a heat transfer surface for heating and cooling,

a fan for air circulation, and a means of cleaning

the air, motor, drive, and casing.

A self-contained air-conditioning unit is

understood to be an air-conditioning unit that is

complete with compressor, condenser,

evaporator, controls, and casing.

7-5

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